Batteries electrode overvoltage

The surface layer formation is a crucial phenomenon as it affects the electrode charge transfer rate. Low-conductivity films determine high electrode overvoltages and lower battery performance. The charge transfer resistance can be monitored by impedance spectroscopy measurements, which represent a useful tool for the in-situ characterization of resistive and capacitive processes occurring in different time scales (1 mHz-100 kHz) [80]. 

In addition to mass transport from the bulk of the electrolyte phase, electroactive material may also be supplied at the electrode surface by homogeneous or heterogeneous chemical reaction. For example, hydrogen ions required in an electrode process may be generated by the dissociation of a weak acid. As this is an uncommon mechanism so far as practical batteries are concerned (but not so for fuel cells), the theory of reaction overvoltage will not be further developed here. However, it may be noted that Tafel-like behaviour and the formation of limiting currents are possible in reaction controlled electrode processes. 

Kinetic processes in an electrochemical reaction must be considered when current is extracted from a battery, that is, when the current passed through an external circuit is different from zero. In this case, while current is removed from the battery, the equilibrium voltage or open circuit voltage falls because of electrode polarization or overvoltage [6,8,10,66,123,124], This electrode polarization effect occurs owing to kinetic limitations experienced by the reactions, and because of other processes taking place during the production of current flow. 

If it were not for the high overvoltage of hydrogen on lead and lead oxide electrodes, the lead/acid storage batteries found in automobiles and trucks (Figure... 

The main component of voltaic inefficiency is usually the ohmic loss, compounded by mass-transport-related overvoltage. The extent of coulombic losses depends on the system and may be due to parasitic electrode and/or chemical reactions, self-discharge, and/or shunt currents (in flowing systems wth a common electrolyte). The efficiency, as measured at battery terminals differs from the effective value if a part of the battery s energy is used to operate auxiliaries (e.g., pumps) or if thermal losses are involved (high-temperature batteries). 

The so-determined Ri comprises ohmic resistance within the electrodes and the electrolyte as well as overvoltage at the phase boundaries between the electrodes and the electrolyte. Equation (56) implies that the overvoltage is comparatively small compared to the ohmic voltage drop. To ensure a certain comparability, the tests are specified for many types of batteries and cell sizes (examples in Ref. 5). 

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